Systems and methods for reducing secondary emissions from catalyst components

ABSTRACT

System and methods for reducing secondary emissions in an exhaust stream from an internal combustion engine are disclosed. The systems and methods include a filtration device positioned downstream from an SCR catalyst of an aftertreatment system disposed in the exhaust system. The filtration device can also be used for particulate filter diagnostics and for treatment of ammonia slip.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a divisional of U.S. patent application Ser.No. 13/551,723 filed on Jul. 18, 2012, which is incorporated herein byreference in its entirety for all purposes.

BACKGROUND

The present application generally relates to exhaust aftertreatmentsystems, and more particularly, but not exclusively, to selectivecatalytic reduction (“SCR”) systems.

Presently available SCR systems adsorb ammonia (NH3) on a catalyst andthen react the NH3 with NOx to reduce the NOx emissions. The NH3 istypically stored as a less reactive composition, e.g. urea, andhydrolyzed into NH3 in the exhaust system as required to reduce the NOxemitted by the engine. At certain system operating conditions, thecatalyst may produce secondary emissions of catalytic material. Thepossibility of emission of catalytic material has led to resistance ofadoption of certain types of SCR catalysts, such as vanadium based SCRcatalysts. In addition, SCR systems are known to create “ammonia slip”in which ammonia that does not adsorb slips through the SCR catalyst.

SCR systems also typically employ a diesel particulate filter upstreamof the SCR catalyst. Available techniques for diesel particulate filter(“DPF”) diagnostics suffer from a number of disadvantages, drawbacks andinadequacies including an inability to adequately diagnose DPF loadingand loss of filtration efficiency among others.

Therefore, a need remains for systems and methods for treating secondaryemissions, including those comprising catalytic material and ammoniaslip. A need also remains for improving diagnostic capabilities ofdiesel particulate filter conditions.

SUMMARY

One embodiment is a unique system, method and device to reduce secondaryemissions from an exhaust of an internal combustion engine. Anotherembodiment is a unique system, method and device to provide an onboarddiagnostic capability for a diesel particulate filter. Furtherembodiments, forms, objects, features, advantages, aspects, and benefitsshall become apparent from the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a system including an exemplaryaftertreatment system.

FIGS. 2A and 2B are schematic illustrations of subsystems included in anexemplary aftertreatment system.

FIG. 3 is a diagram illustrating an exemplary control operation for anaftertreatment procedure.

FIG. 4 is a flow diagram of a procedure that can be performed with anaftertreatment system.

DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended, any alterations and further modificationsin the illustrated embodiments, and any further applications of theprinciples of the invention as illustrated therein as would normallyoccur to one skilled in the art to which the invention relates arecontemplated herein.

With reference to FIG. 1, there is illustrated a system 100 including anengine 110 which is configured to provide rotating mechanical power tosystem 100 and to output exhaust to an exhaust flow path 120. System 100is illustrated schematically and may be included with a car, truck, bus,boat, recreational vehicle, construction equipment or another type ofvehicle. Other embodiments include an engine provided in non-vehicularapplications such as a generator set. The exhaust output by engine 110includes NOx and other components which are to be reduced using anexhaust aftertreatment system 115. In certain implementations, thesystem 100 includes an exhaust gas recirculation (EGR) line (not shown)configured to allow a portion of the exhaust gas generated by the engineto recirculate back into the engine for altering the combustionproperties of the engine 110.

In one embodiment, exhaust aftertreatment system 115 may include anoxidation catalyst 122 which is in fluid communication with exhaust flowpath 120 and is operable to catalyze oxidation of one or more compoundsin exhaust flowing through exhaust flow path 120, for example, oxidationof unburned hydrocarbons or oxidation of NO to NO2. Oxidation catalyst122 can be any of various flow-through oxidation catalysts. Generally,oxidation catalyst 122 includes a substrate with an active catalystlayer configured to oxidize at least some particulate matter (e.g., thesoluble organic fraction of soot) in the exhaust and reduce unburnedhydrocarbons and CO in the exhaust to less environmentally harmfulcompounds. For example, in some implementations, the oxidation catalyst122 may sufficiently reduce the hydrocarbon and CO concentrations in theexhaust to meet the requisite emissions standards.

Exhaust aftertreatment system 115 also includes a diesel particulatefilter 124 in fluid communication with exhaust flow path 120 andoperable to reduce the level of particulates in exhaust flowing throughexhaust flow path 120. In an exemplary embodiment diesel particulatefilter 124 is a catalyzed soot filter. The diesel particulate filter 124can be any of various particulate filters known in the art configured toreduce particulate matter concentrations, e.g., soot and ash, in theexhaust gas to meet requisite emission standards. The diesel particulatefilter 124 includes a filter substrate that captures soot and otherparticulate matter generated by the engine 110. The system 100periodically regenerates diesel particulate filter 124 to removeparticulate matter that has accumulated on the diesel particulate filterover time. For example, diesel particulate filter 124 can be regeneratedby increasing the temperature of the exhaust gas above a thresholdtemperature corresponding with combustion of the particulate matter.

Exhaust aftertreatment system 115 may include a reductant injector 140and an SCR catalyst 130 downstream from particulate filter 124.Reductant injector 140 is supplied with reductant from a reductantreservoir 150 and is operable to inject reductant into exhaust flow path120. In an exemplary embodiment the reductant is an aqueous solution ofurea which decomposes to provide ammonia. Other embodiments utilizedifferent reductants, for example, aqueous solutions of ammonia,anhydrous ammonia, or other reductants suitable for SCR. Reductantinjected into exhaust flow path 120 is provided to SCR catalyst 130which is in flow communication with exhaust flow path 120 and isoperable to catalyze the reduction of NOx. Like the filter substrate ofthe diesel particulate filter 124, the SCR catalyst 130 can be subjectto high temperatures, such as during a regeneration event on the dieselparticulate filter 124.

In certain embodiments, the SCR catalyst 130 includes a vanadium basedcatalyst. Vanadium based SCR systems can be attractive commerciallysince less expensive oxidation catalysts may be employed inaftertreatment system when compared to, for example, zeolite based SCRsystems. However, at high temperatures, such as occur duringregeneration of diesel particulate filter 124, the vanadium based SCRcatalyst may produce secondary emissions of catalytic material. Thus,adoption of vanadium based SCR systems in the United States has beenhampered over environmental concerns associated with secondary emissionsof catalytic material under such high-temperature conditions. For otherembodiments, the SCR catalyst 130 can be any of various catalysts knownin the art. For example, in some implementations, the SCR catalyst is azeolite based catalyst, such as a Cu-Zeolite or a Fe-Zeolite catalyst.

Exhaust aftertreatment system 115 may further include a hydrocarbon (HC)injector 195 which is supplied with HC from an HC reservoir 190 and isoperationally coupled to the exhaust stream at a position upstream ofSCR catalyst 130. HC reservoir may also be coupled to a cylinder 111 ofengine 110. Other embodiments of system 100 may include an engine 110having a common rail fuel system capable of injecting a post injectionfuel where at least a portion of the post injection fuel does notcombust to provide HC in the exhaust stream. Embodiments are alsocontemplated without a HC injector.

Downstream from SCR catalyst 130 there is provided a secondary emissionreduction device 160. In one embodiment, reduction device 160 is afilter or filter substrate arranged to capture secondary emissions from,for example, SCR catalyst 130. Reduction device 160 is positioned at alocation sufficiently spaced from SCR catalyst 130 so that exhauststream temperatures at reduction device 160 are less than at SCRcatalyst 130. Positioning reduction device 160 in a lower temperatureregion of aftertreatment system 115 permits condensation of volatizedcatalyst particles on the filter of reduction device 160. In still otherembodiments, reduction device 160 includes a mesh screen structure orfilter coated with a material that physically and/or chemically enhancescapture and retention of volatized catalyst particles on reductiondevice 160. Examples of suitable coating materials include high surfacearea □-Al2O3 and salts of potassium or sodium.

Each of the catalyst or filter substrates of the oxidation catalyst 122,diesel particulate filter 124, and SCR catalyst 130 are subject to atleast partially fail or deteriorate due to any of various conditions.For example, high exhaust temperature events such as during dieselparticulate filter regeneration, vibratory stresses, and aging ofcomponents can lead to a substrate failure or deterioration where piecesor particles associated with the component are emitted through theexhaust flow. Reduction device 160 captures such pieces and particlesprior to emission into the environment to reduce or eliminate secondaryemissions from one or more of these components.

Exhaust flow path 120, as illustrated schematically in FIG. 1, may beprovided in a variety of physical configurations. In an exemplaryembodiment an exhaust flow path proceeds from the output of aturbocharger (not shown) of engine 110 through a conduit to a structurecontaining oxidation catalyst 122 and diesel particulate filter 124,through a second conduit to a structure containing SCR catalyst 130 andthrough another conduit to reduction device 160, which outlets to theambient environment. Reduction device 160 is a filter in one embodimentthat carries an ammonia oxidation catalyst. Since reduction device 160is located at a position downstream of the SCR catalyst 130, thereduction device 160 can also provide a filtered ammonia oxidationcatalyst which is operable to catalyze the reaction of NH3 which slipsfrom the SCR catalyst 130.

In other embodiments, the components of the exhaust gas after-treatmentsystem 115 can be positioned in any of various arrangements, and thesystem can include other components or fewer components. For example,FIG. 2A illustrates an embodiment of exhaust gas aftertreatment system115 with diesel particulate filter 124, SCR catalyst 130 downstream fromdiesel particulate filter 124, and secondary emission reduction device160 downstream from SCR catalyst 130. Generally, exhaust gas treated inthe exhaust gas after-treatment system 115 and released into theatmosphere consequently contains significantly fewer pollutants, such asdiesel particulate matter, NOx, and hydrocarbons, such as carbonmonoxide and carbon dioxide, than untreated exhaust gas.

Referring back to system 100 in FIG. 1, in certain embodiments, system100 includes a controller 180 which functionally executes certainoperations for engaging aftertreatment system 115 and/or system 100.Controller 180 forms a portion of a processing subsystem including oneor more computing devices having memory as well as a number of inputsand outputs for interfacing with various sensors and systems of system100. Controller 180 can be an electronic circuit comprised of one ormore components, including digital circuitry, analog circuitry, or both.Controller 180 may be a single device or a distributed device.Controller 180 may include one or more control algorithms defined byoperating logic in the form of software instructions, hardwareinstructions, firmware instructions, dedicated hardware, or the like.

In one form, controller 180 is of a programmable microcontrollersolid-state integrated circuit type that includes memory and one or morecentral processing units. The memory of controller 180 includes of oneor more components and can be of any of volatile or nonvolatile,solid-state, optical media, magnetic media, combinations of these, orother types of memory. Controller 180 can include signal conditioners,signal format converters (such as analog-to-digital anddigital-to-analog converters), limiters, clamps, filters, and the likeas needed to perform various control and regulation operations describedherein. Controller 180, in an exemplary embodiment, may be a type ofcontroller sometimes referred to as an electronic or engine controlmodule (ECM), electronic or engine control unit (ECU) or the like, thatis directed to the regulation and control of engine operation.Alternatively, controller 180 may be dedicated to the control of justthe operations described herein or to a subset of controlled aspects ofsystem 100.

Controller 180 is in operative communication with a differentialpressure sensor 170 (FIG. 2B) which provides controller 180 withinformation indicative of the pressure drop across reduction device 160.In a further embodiment, controller 180 is in operative communicationwith a differential pressure sensor 175 (FIG. 2B) which providescontroller 180 with information indicative of the pressure drop acrossdiesel particulate filter 124. Controller 180 may also be in operativecommunication with one or more temperature sensors that indicatetemperature of the exhaust system. In other embodiments, informationfrom temperature sensors, flow sensors, pressure sensors, and NOxsensors in various locations is utilized to determine informationindicative of the conditions of SCR catalyst 130 and/or the exhaustsystem. Controller 180 may also be in operative communication with HCinjector 195 and HC reservoir 190, and/or reductant injector 140 andreservoir 150 for treatment of exhaust gases as known in the art.

Differential pressure sensor 170 is fluidly coupled to the exhaustflowpath 120 at a first position upstream of the reduction device 160and at a second position downstream of reduction device 160. A seconddifferential pressure sensor 175 can be fluidly coupled to the exhaustflowpath 120 at a first position upstream of diesel particulate filter124 and at a second position downstream of the diesel particulate filter124. The differential pressure sensors 170, 175 may be a single pressuretransducer, multiple pressure transducers, a single electromechanicalpressure sensor, two inductive pressure sensors or any other combinationof pressure sensor(s) that can be configured to determine a pressuredrop across the reduction device 160 and/or diesel particulate filter124. This pressure drop may be conveyed from the differential pressuresensor 170, 175 as a pressure value, multiple pressure values where adifference can be taken, a voltage which may be converted to a pressurevalue, and/or a digital signal which can be read by a processor orprocessor subsystem and is correlated to a pressure value.

Controller 180 is operable to determine if a pressure drop of theexhaust gas stream across reduction device 160 is indicative of afailure of diesel particulate filter 124. For example, during normaloperation with an operations particulate filter 124, the pressure dropacross reduction device 160 is relatively small since diesel particulatefilter 124 filters the particulate matter from the exhaust stream. Afailure of particulate filter 124 causes reduction device 160 to receivethe particulates from the exhaust stream, thus causing a substantialincrease in the pressure drop across reduction device 160. When pressuresensor 170 provides an indication of that the pressure drop acrossreduction device 160 has increased by more than a predeterminedthreshold, then controller 180 is operable to determine if the pressurechange is indicative of a failure of diesel particulate filter 124 andprovide a communication to an onboard diagnostic system regarding thesame.

Controller 180 may also be operable to determine if a pressure drop ofthe exhaust gas stream across diesel particulate filter 124 indicated bydifferential pressure sensor 175 is indicative of a failure of dieselparticulate filter 124. Controller 180 can be operatively linked to anonboard diagnostics system to provide an indication of dieselparticulate filter failure based on one or both of the pressure dropsignals provided by sensor 170 and/or sensor 175.

In certain embodiments, the controller 180 includes one or more modulesstructured to functionally execute the operations of the controller 180.The description herein including modules emphasizes the structuralindependence of the aspects of the controller, and illustrates onegrouping of operations and responsibilities of the controller 180. Othergroupings that execute similar overall operations are understood withinthe scope of the present application. Modules may be implemented inhardware and/or software on computer readable medium, and modules may bedistributed across various hardware or software components.

Controller 180 is in operative interconnection with various elements ofsystem 100 as illustrated in FIG. 1 with dashed lines extending betweencontroller 180 and various elements of system 100. These operativeinterconnections may be implemented in a variety of forms, for example,through input/output interfaces coupled via wiring harnesses, adatalink, a hardwire or wireless network and/or a lookup from a memorylocation. In other instances all or a portion of the operativeinterconnection between controller 180 and an element of system 100 maybe virtual. For example, a virtual input indicative of an operatingparameter may be provided by a model implemented by controller 180 or byanother controller which models an operating parameter based upon otherinformation.

FIG. 3 represents an apparatus 200 that includes controller 180 withvarious components illustrated as representative modules, inputs,outputs, and intermediate data parameters. According to one embodiment,the controller 180 includes a particulate filter failure detectionmodule 210 configured to determine a failure condition of dieselparticulate filter 124. For example, the particulate filter failurecondition module 210 may be configured to determine a failure conditionfor the diesel particulate filter 124.

The particulate filter failure detection module 210 can use any ofvarious methods and techniques for determining the particulate matterloads associated with diesel particulate filter 124. For example, incertain implementations, the controller 180 receives input from thedifferential pressure sensor 170 indicating a pressure difference acrossreduction device 160. In further implementations, controller 180 alsoreceives input from the differential pressure sensor 175 across dieselparticulate filter 124. Additionally, the controller 180 may receiveother input regarding any of various conditions of the system 100, suchas exhaust flow rates, exhaust temperatures, exhaust component massconcentrations, from other sensors 220. In one specific implementation,based on the input from the differential pressure sensor 170, therespective exhaust gas flow rates through the diesel particulate filter124, and/or the other sensed operating conditions, the particulatefilter failure detection module 210 can determine a failure conditionfor diesel particulate filter 124. In another specific implementation,based on the input from the differential pressure sensor 170 anddifferential pressure sensor 175, the respective exhaust gas flow ratesthrough the diesel particulate filter 124, and/or the other sensedoperating conditions, the particulate filter failure detection module210 can determine a failure condition for diesel particulate filter 124.

A failure condition for diesel particulate filter 124 can be triggeredin any of various ways. For example, a determination by the particulatefilter failure module 210 of a substantial increase in the pressure dropacross reduction device 160 indicates that reduction device 160 isaccumulating particles from the exhaust gas normally captured by dieselparticulate filter 124. Therefore, a pressure drop increase acrossreduction device 160 can be measured by sensor 170 and provide anindication that diesel particulate filter 124 is not properlyfunctioning to filter particulates from the exhaust gas when thepressure drop increase exceeds a predetermined threshold.

In another example, a failure event can be registered based on aparticulate matter load estimate determined by the particulate filterfailure module 210 after a regeneration event. More specifically, aparticulate matter load estimate determines that a pressure drop acrossdiesel particulate filter 124 exceeds a predetermined threshold after aregeneration event such that regeneration is not sufficient to providean operational diesel particulate filter 124.

Alternatively, or additionally, in certain embodiments, the failureevent can be determined by a differential pressure across dieselparticulate filter 124 falling below a predetermined threshold,indicating the lack of an operational filtering element in exhaust flowpath 120. This determination can be coupled with a determination of anincreased pressure differential across secondary device 160 indicatingthat secondary device 160 is being loaded with particles from theexhaust stream normally captured by diesel particulate filter 124. Inone particular embodiment, a first absolute pressure sensor upstream ofdiesel particulate filter 124 and a second absolute pressure sensorbetween diesel particulate filter 124 and reduction device 160 provide adiesel filter diagnostic capability. For example, a determination that apressure difference between the two absolute pressure sensors is lessthan a predetermined threshold can provide an indication that dieselparticulate filter 124 is non-operational due the lack of back pressurebeing created by diesel particulate filter 124.

The failure of diesel particulate filter 124 can be communicated bycontroller 180 to an onboard diagnostics system (not shown) via OBDcommands 230. The OBD commands 230 provide a signal to the operator,service personnel, or others that a service condition for dieselparticulate filter 124 exists.

The schematic flow diagram and related description which followsprovides an illustrative embodiment of performing procedures forengaging a diesel particulate filter diagnostic system. Operationsillustrated are understood to be exemplary only, and operations may becombined or divided, and added or removed, as well as re-ordered inwhole or part. Certain operations illustrated may be implemented by acomputer executing a computer program product on a computer readablemedium, where the computer program product comprises instructionscausing the computer to execute one or more of the operations, or toissue commands to other devices to execute one or more of theoperations.

Referencing FIG. 4, a process 300 includes an operation 310 to provide asecondary emission reduction device downstream of a selective catalyticreduction (SCR) catalyst as part of an exhaust stream of an engine. Thesecondary emission reduction device can be, as discussed above, a filterthat also includes a coating of any known material suitable as anammonia oxidation catalyst. The process 300 further includes anoperation 320 in which the pressure drop across the secondary emissionreduction device is measured at periodic intervals.

Conditional 330 determines if a pressure drop across reduction device160 is more than a pressure drop threshold indicative of failure ofparticulate filter 124. During normal operation of aftertreatment system115, the pressure drop across reduction device 160 is relatively smallsince particulate filter 124 has removed particulates from the exhauststream. A failure of particulate filter 124 permitting particulates topass therethrough results in particulates being accumulated on reductiondevice 160, thereby causing a pressure drop increase across reductiondevice 160. Conditional 330 determines if the pressure drop increase isgreater than a predetermined threshold, where the predeterminedthreshold is greater than a range of pressure drops across reductiondevice 160 expected during normal operation. These values may be setvalues in a look up table and/or may be determined by a calculation withretrieved variables.

If the response to conditional 330 is a YES, then procedure 300continues at operation 340. At operation 340 a signal indicating afailure or service condition for particulate filter 124 is provided toan onboard diagnostics system of the vehicle. If the response toconditional 330 is a NO, then procedure 300 ends until started againafter lapse of a predetermined amount of time and/or occurrence of oneor more operating conditions.

As is evident from the figures and text presented above, a variety ofembodiments according to the present invention are contemplated.

One embodiment is a method, including: (1) providing a selectivecatalytic reduction (SCR) catalyst disposed in an exhaust stream of aninternal combustion engine; (2) providing a diesel particulate filterupstream of the SCR catalyst; (3) providing a secondary emissionreduction device downstream of the SCR catalyst; (4) determining that apressure drop across the secondary emission reduction device exceeds apredetermined threshold; and (5) providing a signal indicating failureof the diesel particulate filter when the pressure drop exceeds thethreshold.

As a further feature, determining that the pressure drop of the exhauststream across the secondary emission device exceeds the threshold mayinclude: measuring the differential pressure across the secondaryemission reduction device; measuring the absolute pressure of theexhaust steam upstream of the diesel particulate filter and between thediesel particulate filter and the secondary emission reduction device;and measuring the pressure drop across the diesel particulate filter.

Another embodiment includes a system with a selective catalyticreduction (SCR) catalyst disposed in an exhaust stream of an internalcombustion engine; a diesel particulate filter in the exhaust streamupstream from the SCR catalyst; and a secondary emission reductiondevice in the exhaust stream downstream from the SCR catalyst. In oneform, the secondary emission reduction device includes a filter tocapture secondary emissions from a vanadium based SCR catalyst. Thefilter may include one or more coatings or surface area treatments tophysically or chemically enhance capture and retention of volatizedcatalyst components. In another form, the secondary emission reductiondevice includes a filter that is a carrier for an ammonia slip catalyst.

Another embodiment of the system includes a controller structured todetermine that a failure condition for the diesel particulate filter hasoccurred. This embodiment may further include the controller providingan onboard diagnostic signal indicative of the failure.

The controller of this embodiment may be further structured to determinethat a pressure drop of the exhaust stream across a secondary emissionreduction device is more than a threshold indicative of a dieselparticulate filter failure upstream of the secondary device.

Further features of this embodiment may include the afterteatment systemhaving a differential pressure sensor at the secondary emissionreduction device. In a further refinement, a differential pressuresensor is provided at the diesel particulate filter.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly certain exemplary embodiments have been shown and described andthat all changes and modifications that come within the spirit of theinventions are desired to be protected. In reading the claims, it isintended that when words such as “a,” “an,” “at least one,” or “at leastone portion” are used there is no intention to limit the claim to onlyone item unless specifically stated to the contrary in the claim. Whenthe language “at least a portion” and/or “a portion” is used the itemcan include a portion and/or the entire item unless specifically statedto the contrary.

What is claimed is:
 1. A method, comprising: providing a selectivecatalytic reduction (SCR) catalyst disposed in an exhaust stream of aninternal combustion engine; providing a diesel particulate filter in theexhaust stream upstream of the SCR catalyst; providing a secondaryemission reduction device in the exhaust stream downstream of the SCRcatalyst; determining that a pressure drop across the secondary emissionreduction device exceeds a first predetermined threshold; determiningthat a second pressure drop in the exhaust stream across the dieselparticulate filter is less than a second predetermined threshold; andproviding a signal indicating failure of the diesel particulate filterfrom the second pressure drop across the diesel particulate filter beingless than the second predetermined threshold in addition to the pressuredrop across the secondary emission reduction device exceeding the firstpredetermined threshold.
 2. The method of claim 1, wherein the SCRcatalyst is a vanadium based catalyst.
 3. The method of claim 2, whereinthe secondary emission reduction device is a filter.
 4. The method ofclaim 3, wherein the filter carries an ammonia oxidation catalyst. 5.The method of claim 4, wherein the filter of the secondary emissionreduction device includes a material coating to enhance filtering ofvolatized SCR catalyst components.
 6. The method of claim 1, furthercomprising providing an onboard diagnostic command corresponding to thefailure of the diesel particulate filter.
 7. A method, comprising:providing a selective catalytic reduction (SCR) catalyst disposed in anexhaust stream of an internal combustion engine, wherein the SCRcatalyst is a vanadium based catalyst; providing a diesel particulatefilter in the exhaust stream upstream of the SCR catalyst; providing asecondary emission reduction device in the exhaust stream downstream ofthe SCR catalyst; determining that a pressure drop across the secondaryemission reduction device exceeds a first predetermined threshold; anddetermining that a second pressure drop across the diesel particulatefilter is less than a second predetermined threshold in addition todetermining the pressure drop across the secondary emission reductiondevice exceeds the first predetermined threshold in order to provide anonboard diagnostic command corresponding to the failure of the dieselparticulate filter.
 8. The method of claim 1, wherein the dieselparticulate filter failure corresponds to an inability of the dieselparticulate filter to remove particulates from the exhaust stream. 9.The method of claim 1, wherein the secondary emission reduction deviceis in a temperature region of the exhaust stream that is lower than thatof the SCR catalyst so that volatized catalytic material from the SCRcatalyst condenses on the filter.
 10. The method of claim 1, wherein thesecondary emission reduction device includes an ammonia oxidationcatalyst to catalyze ammonia slip from the SCR catalyst.
 11. The methodof claim 1, wherein the secondary emission reduction device includes afilter having a material coating to enhance filtering of volatized SCRcatalyst components.
 12. The method of claim 11, wherein the materialcoating includes at least one of sodium, potassium or γ-Al₂O₃.
 13. Themethod of claim 7, wherein the secondary emission reduction device is afilter.
 14. The method of claim 13, wherein the filter carries anammonia oxidation catalyst.
 15. The method of claim 14, wherein thefilter of the secondary emission reduction device includes a materialcoating to enhance filtering of volatized SCR catalyst components. 16.The method of claim 7, wherein the diesel particulate filter failurecorresponds to an inability of the diesel particulate filter to removeparticulates from the exhaust stream.
 17. The method of claim 7, whereinthe secondary emission reduction device is in a temperature region ofthe exhaust stream that is lower than that of the SCR catalyst so thatvolatized catalytic material from the SCR catalyst condenses on thefilter.
 18. The method of claim 7, wherein the secondary emissionreduction device includes an ammonia oxidation catalyst to catalyzeammonia slip from the SCR catalyst.
 19. The method of claim 7, whereinthe secondary emission reduction device includes a filter having amaterial coating to enhance filtering of volatized SCR catalystcomponents.
 20. The method of claim 19, wherein the material coatingincludes at least one of sodium, potassium or γ-Al₂O₃.